Light emitting diodes (LED's) are frequently used as the source of light for lasers. Typically, thousands of LED's are required to effectively power a laser. To minimize the size of all of these diodes, a single GaAs bar is doped in a manner to produce 40 to 400 LED's in the bar and then the bars are stacked on top of each other creating a "lighting rod".
The laser diode bars can generate average heat fluxes ranging from 5 W/cm.sup.2 to 125 W/cm.sup.2 and must operate at or below 25.degree. C. To ensure that the temperature of the diodes do not exceed 25.degree. C., the diodes are directly mounted to a cooler capable of removing large amounts of heat, without producing large temperature differentials between the coolant of the cooler and the bar. The cooler should be thin in the range of 0.150 cm, to allow the bars and coolers to stack on top of each other, and have low coolant passage pressure drops such that the cooler can used with standard chillers.
It is essential to have all of the diodes operating within 1.degree. C. of each other, so that the diodes produce light at the same wavelength. Therefore, the cooler should not only be thin and have high heat transfer coefficients, but should also produce an isothermal mounting surface.
McLafferty et al., U.S. Pat. No. 3,637,296, discloses a thin heat cooler with cooling passages in close proximity to the heating surface, where the cooling fluid flows through all the passages in the same direction, which effectively cools the mounting surface but produces a temperature gradient along the heating surface.
Sorensen et al., U.S. Pat. No. 3,708,223, reveals a thin cooler with cooling passages in close proximity to the mounting surface. The coolant flows through adjacent passages in opposite directions to balance the temperature of the heating surface along the length of the passages.
Dunn III et al., U.S. Pat. No. 3,884,558, discloses a thin cooler with a plurality of first cooling passages in close proximity to the mounting surface. The coolant flows in opposite directions in adjacent first passages and then passes through a plurality of second passages oriented perpendicular to the first passages. Passing the coolant through the second passages further equalizes the temperature throughout the heating surface.
Schmidt et al., U.S. Pat. No. 3,909,118, and the article, Bland, Niooemann and Parekh. "A Compact High Intensity Cooler (Chic)", SAE Technical Paper Series 831127, July 11-13, 1983, discloses a thin jet impingment cooler, where the coolant passages are perpendicular to the heating surface. The orientation of the passages allows the coolant to thermally scrub the back surface of the mounting plate, producing an even temperature gradient throughout the mounting plate.
Tuckerman and Pease, "High-Performance Heat Sinking for VSLI", IEEE Electron Device Letters, Vol. EDL-2, No. 5, May, 1981, discusses a cooler that accomplishes high forced convection heat transfer coefficients by providing a plurality of very small cooling passages.
Of the patents and publications discussed, only the "Chic" cooler appeared to combine a thin width with high heat transfer coefficients and an isothermal mounting surface. Yet when the "Chic" cooler was tested with a heat flux of 125 W/cm.sup.2 it produced temperature gradients of 3.5.degree. C., a result which did not comply with the laser requirement of 1.degree. C.